The present invention relates to a cold trap for liquid metal, such as is found in the nuclear industry, as well as to processes for using this trap.
Fast neutron nuclear reactors use liquid sodium as the cooling fluid, both for the primary circuit and for the secondary circuits. The length of the operating cycles, as well as their number, explains why it gradually becomes charged with impurities, mainly sodium hydride and oxide. These pollutants can be continuously produced during the operation of the power station, e.g. by hydrogen diffusion through the steam generator walls to the secondary circuit, or by the activation of steel or by diffusion through the fuel cans of certain fission products, such as protons and tritium in the case of the primary circuit. The pollutants can also be discontinuous. Handling undergone by the assemblies during each cycle and also occasionally certain components, pollute the sodium by introducing adsorbed gases and metal oxides. Finally, steam generator leaks can lead to sodium-water reactions in the secondary circuit, with the production of sodium hydroxide and hydrogen, which can decompose into hydride and oxide.
This leads to the corrosion of the metal surfaces of the circuits by hydrogen and to the embrittlement thereof by the hydrogen, the pollution thereof by corrosion products carried by the sodium and activated in the core and deposited in the cold zones which are consequently contaminated. This can cause handling and maintenance problems, disturbance to the detection measurements of steam generator leaks in the secondary circuit through variations in the hydrogen concentration and finally to the crystallization of the sodium hydride and oxide at the cold points of the circuit, which can lead to the clogging of the narrowed sections or can damage the pumps.
It is to eliminate such pollutants that the sodium circuits are provided with cold traps, which essentially comprise a reservoir which is cooled in such a way that the impurities crystallize and are held back in said traps. To this end, it is possible to use a retention element constituted by metal wool through which the sodium is forced to pass. Thus, the outflow, which becomes turbulent, and the presence of a tight network of wires assists the crystal germination effect. The satisfactory operation of the reactor requires, on the one hand, the permanent use of a cold trap under so-called "maintenance purification" conditions, because the impurities created by the continuous pollution must be fixed with a speed equal to the pollution speed and, on the other hand, occasionally, to a more intense use of the trap, which is obtained by increasing the flow rate during a "purification campaign", in order to reduce the concentration of oxygen and hydrogen due to a discontinuous pollution. Several problems are caused by the presently known cold traps. Firstly the impurity retention element is liable to rapid dirtying and this can lead to the clogging thereof making it necessary to change said trap several times during the life of the power station.
In addition, the volume of the cold trap is not very well used, because the entry temperature into the trap is above the temperature at which crystals appear for the corresponding concentration of impurities. Thus, part of the trap length, located close to the intake remains unused. This unused trap length is greater under maintenance purification conditions than during the purification campaign. Thus, the prior art traps all operate with a constant longitudinal thermal gradient and the variation between the temperature at which the crystals appear and that of the cold point is smaller under maintenance conditions than during a purification campaign.